The invention is based on a method for controlling the power of a high-pressure gas discharge lamp to claim 1.
U.S. Pat. No. 4,240,009 discloses a circuit for operating a gas-discharge lamp that uses a characteristic curve to determine the power supplied to the lamp. The control device includes a power-supply circuit. This circuit is controlled and regulated by means of the voltage/current characteristic curve, so that the lamp is maintained at a specific, particularly constant, power during operation. In this known circuit, the power is determined with a wattmeter, which uses applied voltage and flowing current to determine the power supplied to a regulated inverter circuit by a battery, and from the power, generates control signals for the inverter circuit corresponding to the predetermined voltage/current characteristic curve. If constant power is maintained for the lamp, the respective operating point lies on the so-called power hyperbola.
To provide control during startup and during operation of a high-pressure gas discharge lamp, the Vedilis curve is generally predetermined as a current/voltage curve in the sense of a nominal-value curve. The Vedilis curve is represented in the "System Specifications for Field Test" of the VEDILIS Eureka Project 273, p. B 1/3, and predetermined as a current/voltage characteristic curve for gas discharge lamps to be used in motor vehicles. "Vedilis" is an acronym for "Vehicle Discharge Light System." Accordingly, to regulate the lamp power of a gas discharge lamp, the lamp voltage U is measured during the startup or burning phase, for example, and the corresponding lamp current I associated with the respective lamp voltage U is determined from the Vedilis curve. This lamp current I then serves as a nominal value for the current-regulating circuit with which the lamp power is regulated at a constant value, for example 35 W.
In general, for using a gas discharge lamp in a motor-vehicle headlight, it is necessary to make available as much light as possible within a short time after the control device has been initiated. This is referred to as rapid light startup. As the aforementioned Vedilis curve indicates, the gas discharge lamp can be operated at a certain overload immediately after being turned on. This overload is then reduced after a certain voltage has been attained, as a function of the lamp voltage. This reduction occurs along the hyperbolic portion of the characteristic curve. It is clear that this type of overload must not be allowed to destroy the lamp or negatively affect its service life. This requirement exists for all operating conditions. Furthermore, the control is to be selected such that the light power does not drop back below a value that has already been attained, because this is generally perceived as annoying flickering. This drop in light power is also called light saddle.